Depending on the bank/brand of card, the security code should
be printed on the back, and like a credit card appear in a box
with contrasting color. Some cards include the last four digits
of the main number on the front (that may not be needed when
only security code is asked for, depending on vendor/purpose)
along with a three digit security code.
So, say for mastercharge the last four card numbers may appear
in front of the three digit security code, on the back of the card.
Questions about this kind of thing should be directed at the Bank
whose main credit affiliation is noted on the card, to tell you where
to look, should the general description given here not be enough.
Good luck & happy computing! 🙂